In wireless communications, information is transmitted from a source node to a destination node over the air. The larger the transmit power at the source node, the larger the possible distance between the source node and destination node. However, the relationship between transmit-power and distance is non-linear. Doubling the separation between the source node and the destination node requires more than a doubling of transmit-power at the source node in order to receive a similar signal level at the destination node.
Placing a third node, a relay node, in between the source node and destination node enables the single long-range transmission hop from source to destination to be substituted by two shorter transmission hops, i.e. from the source node to the relay node, and subsequently, from the relay node to the destination node.
In spite of the additional delay incurred by relaying the data (i.e. using two hops instead of a single hop), due to the non-linear relationship mentioned above, the energy savings gained by replacing a single large hop with two shorter-haul transmissions could nonetheless be beneficial, especially when considered on a system-wide scale.
Therefore, in a wireless network comprising of multiple nodes, for any communication between a source node and a destination node, a third node may be enlisted to act as a relay node in order to improve the energy efficiency of the entire system.
The problem then faced is how to determine which of the other nodes in the system, if any, could act as a relay node for an ongoing communication between a source node and a destination node.
WO2010/006649 discloses a cellular system, allowing direct device-to-device communication between terminal devices, and also allowing a controller to select some of the terminal devices to act as relays to enable relayed communication links between other terminal devices and the base station. The controller determines a number of relay terminals required in a cell, and terminal devices are then selected to act as relays based on the channel conditions between the base station and the respective terminal devices.
However, this system does not provide the best possible selection of a relay device, particularly in the case of device-to-device communication between terminal devices without passing through the base station.